A river is a natural flowing watercourse, usually freshwater, flowing
towards an ocean, sea, lake or another river. In some cases a river
flows into the ground and becomes dry at the end of its course without
reaching another body of water. Small rivers can be referred to using
names such as stream, creek, brook, rivulet, and rill. There are no
official definitions for the generic term river as applied to
geographic features,[1] although in some countries or communities a
stream is defined by its size. Many names for small rivers are
specific to geographic location; examples are "run" in some parts of
the United States, "burn" in Scotland and northeast England, and
"beck" in northern England. Sometimes a river is defined as being
larger than a creek,[2] but not always: the language is vague.[3]
Rivers are part of the hydrological cycle. Water generally collects in
a river from precipitation through a drainage basin from surface
runoff and other sources such as groundwater recharge, springs, and
the release of stored water in natural ice and snowpacks (e.g., from
glaciers). Potamology is the scientific study of rivers, while
limnology is the study of inland waters in general.

A river begins at a source (or more often several sources), follows a
path called a course, and ends at a mouth or mouths. The water in a
river is usually confined to a channel, made up of a stream bed
between banks. In larger rivers there is often also a wider floodplain
shaped by flood-waters over-topping the channel. Floodplains may be
very wide in relation to the size of the river channel. This
distinction between river channel and floodplain can be blurred,
especially in urban areas where the floodplain of a river channel can
become greatly developed by housing and industry.
Rivers can flow down mountains, through valleys (depressions) or along
plains, and can create canyons or gorges.
The term upriver (or upstream) refers to the direction towards the
source of the river, i.e. against the direction of flow. Likewise, the
term downriver (or downstream) describes the direction towards the
mouth of the river, in which the current flows.
The term left bank refers to the left bank in the direction of flow,
right bank to the right.
The river channel typically contains a single stream of water, but
some rivers flow as several interconnecting streams of water,
producing a braided river.[4] Extensive braided rivers are now found
in only a few regions worldwide,[citation needed] such as the South
Island of New Zealand. They also occur on peneplains and some of the
larger river deltas. Anastamosing rivers are similar to braided rivers
and are quite rare. They have multiple sinuous channels carrying large
volumes of sediment. There are rare cases of river bifurcation in
which a river divides and the resultant flows ending in different
seas. An example is the bifurcation of Nerodime
RiverRiver in Kosovo.

A river flowing in its channel is a source of energy which acts on the
river channel to change its shape and form. In 1757, the German
hydrologist
Albert Brahms empirically observed that the submerged
weight of objects that may be carried away by a river is proportional
to the sixth power of the river flow speed.[5] This formulation is
also sometimes called Airy's law.[6] Thus, if the speed of flow is
doubled, the flow would dislodge objects with 64 times as much
submerged weight. In mountainous torrential zones this can be seen as
erosion channels through hard rocks and the creation of sands and
gravels from the destruction of larger rocks. A river valley that was
created from a U-shaped glaciated valley, can often easily be
identified by the V-shaped channel that it has carved. In the middle
reaches where a river flows over flatter land, meanders may form
through erosion of the river banks and deposition on the inside of
bends. Sometimes the river will cut off a loop, shortening the channel
and forming an oxbow lake or billabong. Rivers that carry large
amounts of sediment may develop conspicuous deltas at their mouths.
Rivers whose mouths are in saline tidal waters may form estuaries.
Throughout the course of the river, the total volume of water
transported downstream will often be a combination of the free water
flow together with a substantial volume flowing through sub-surface
rocks and gravels that underlie the river and its floodplain (called
the hyporheic zone). For many rivers in large valleys, this unseen
component of flow may greatly exceed the visible flow.
Subsurface streams
Most but not all rivers flow on the surface. Subterranean rivers flow
underground in caves or caverns. Such rivers are frequently found in
regions with limestone geologic formations. Subglacial streams are the
braided rivers that flow at the beds of glaciers and ice sheets,
permitting meltwater to be discharged at the front of the glacier.
Because of the gradient in pressure due to the overlying weight of the
glacier, such streams can even flow uphill.
Permanence of flow
Main article: Intermittent river
An intermittent river (or ephemeral river) only flows occasionally and
can be dry for several years at a time. These rivers are found in
regions with limited or highly variable rainfall, or can occur because
of geologic conditions such as a highly permeable river bed. Some
ephemeral rivers flow during the summer months but not in the winter.
Such rivers are typically fed from chalk aquifers which recharge from
winter rainfall. In England these rivers are called bournes and give
their name to places such as
BournemouthBournemouth and Eastbourne. Even in humid
regions, the location where flow begins in the smallest tributary
streams generally moves upstream in response to precipitation and
downstream in its absence or when active summer vegetation diverts
water for evapotranspiration. Normally-dry rivers in arid zones are
often identified as arroyos or other regional names.
The meltwater from large hailstorms can create a slurry of water, hail
and sand or soil, forming temporary rivers.[7]
Classification

NileNileRiverRiver delta, as seen from Earth orbit. The
NileNile is an example of
a wave-dominated delta that has the classic Greek letter delta (Δ)
shape after which river deltas were named.

A radar image of a 400-kilometre (250 mi) river of methane and
ethane near the north pole of Saturn's moon Titan

Rivers have been classified by many criteria including their
topography, their biotic status, and their relevance to white water
rafting or canoeing activities.
Topographical classification
Rivers can generally be classified as either alluvial, bedrock, or
some mix of the two. Alluvial rivers have channels and floodplains
that are self-formed in unconsolidated or weakly consolidated
sediments. They erode their banks and deposit material on bars and
their floodplains. Bedrock rivers form when the river downcuts through
the modern sediments and into the underlying bedrock. This occurs in
regions that have experienced some kind of uplift (thereby steepening
river gradients) or in which a particular hard lithology causes a
river to have a steepened reach that has not been covered in modern
alluvium. Bedrock rivers very often contain alluvium on their beds;
this material is important in eroding and sculpting the channel.
Rivers that go through patches of bedrock and patches of deep alluvial
cover are classified as mixed bedrock-alluvial.
Alluvial rivers can be further classified by their channel pattern as
meandering, braided, wandering, anastomose, or straight. The
morphology of an alluvial river reach is controlled by a combination
of sediment supply, substrate composition, discharge, vegetation, and
bed aggradation.
At the start of the 20th century
William Morris DavisWilliam Morris Davis devised the
"cycle of erosion" method of classifying rivers based on their "age".
Although Davis's system is still found in many books today, after the
1950s and 1960s it became increasingly criticized and rejected by
geomorphologists. His scheme did not produce testable hypotheses and
was therefore deemed non-scientific.[8] Examples of Davis's river
"ages" include:

Youthful river: A river with a steep gradient that has very few
tributaries and flows quickly. Its channels erode deeper rather than
wider. Examples are the Brazos, Trinity and Ebro rivers.
Mature river: A river with a gradient that is less steep than those of
youthful rivers and flows more slowly. A mature river is fed by many
tributaries and has more discharge than a youthful river. Its channels
erode wider rather than deeper. Examples are the Mississippi, Saint
Lawrence, Danube, Ohio, Thames and Paraná rivers.
Old river: A river with a low gradient and low erosive energy. Old
rivers are characterized by flood plains. Examples are the Yellow,
lower Ganges, Tigris, Euphrates, Indus and lower
NileNile rivers.
Rejuvenated river: A river with a gradient that is raised by tectonic
uplift. Examples are the
Rio GrandeRio Grande and Colorado River.

The ways in which a river's characteristics vary between its upper and
lower course are summarized by the Bradshaw model. Power-law
relationships between channel slope, depth, and width are given as a
function of discharge by "river regime".
Biotic classification
There are several systems of classification based on biotic conditions
typically assigning classes from the most oligotrophic or unpolluted
through to the most eutrophic or polluted.[9] Other systems are based
on a whole eco-system approach such as developed by the New Zealand
Ministry for the Environment.[10] In Europe, the requirements of the
Water Framework Directive has led to the development of a wide range
of classification methods including classifications based on fishery
status[11] A system of river zonation used in francophone
communities[12][13] divides rivers into three primary zones:

The crenon is the uppermost zone at the source of the river. It is
further divided into the eucrenon (spring or boil zone) and the
hypocrenon (brook or headstream zone). These areas have low
temperatures, reduced oxygen content and slow moving water.
The rhithron is the upstream portion of the river that follows the
crenon. It has relatively cool temperatures, high oxygen levels, and
fast, turbulent, swift flow.
The potamon is the remaining downstream stretch of river. It has
warmer temperatures, lower oxygen levels, slow flow and sandier
bottoms.

WhitewaterWhitewater classification
The
International Scale of River DifficultyInternational Scale of River Difficulty is used to rate the
challenges of navigation—particularly those with rapids. Class I is
the easiest and Class VI is the hardest.
StreamStream order classification
The Strahler
StreamStream Order ranks rivers based on the connectivity and
hierarchy of contributing tributaries. Headwaters are first order
while the
Amazon RiverAmazon River is twelfth order. Approximately 80% of the
rivers and streams in the world are of the first and second order.
In certain languages, distinctions are made among rivers based on
their stream order. In French, for example, rivers that run to the sea
are called fleuve, while other rivers are called rivière. For
example, in Canada, the Churchill
RiverRiver in
ManitobaManitoba is called la
rivière Churchill as it runs to Hudson Bay, but the Churchill River
in
LabradorLabrador is called le fleuve Churchill as it runs to the Atlantic
Ocean. As most rivers in France are known by their names only without
the word rivière or fleuve (e.g. la Seine, not le fleuve Seine, even
though the
SeineSeine is classed as a fleuve), one of the most prominent
rivers in the
FrancophonieFrancophonie commonly known as fleuve is le fleuve
Saint-Laurent (the Saint Lawrence River).
Since many fleuves are large and prominent, receiving many
tributaries, the word is sometimes used to refer to certain large
rivers that flow into other fleuves; however, even small streams that
run to the sea are called fleuve (e.g. fleuve côtier, "coastal
fleuve").
Uses

Leisure activities on the
RiverRiver Avon at Avon
ValleyValley Country Park,
Keynsham, United Kingdom. A boat giving trips to the public passes a
moored private boat.

Rivers have been used as a source of water, for obtaining food, for
transport, as a defensive measure, as a source of hydropower to drive
machinery, for bathing, and as a means of disposing of waste.
Rivers have been used for navigation for thousands of years. The
earliest evidence of navigation is found in the Indus Valley
Civilization, which existed in northwestern India around 3300 BC.[14]
Riverine navigation provides a cheap means of transport, and is still
used extensively on most major rivers of the world like the Amazon,
the Ganges, the Nile, the Mississippi, and the Indus. Since river
boats are often not regulated, they contribute a large amount to
global greenhouse gas emissions, and to local cancer due to inhaling
of particulates emitted by the transports.[15][16]
In some heavily forested regions such as
ScandinaviaScandinavia and Canada,
lumberjacks use the river to float felled trees downstream to lumber
camps for further processing, saving much effort and cost by
transporting the huge heavy logs by natural means.
Rivers have been a source of food since pre-history.[17] They are
often a rich source of fish and other edible aquatic life, and are a
major source of fresh water, which can be used for drinking and
irrigation. Most of the major cities of the world are situated on the
banks of rivers. Rivers help to determine the urban form of cities and
neighbourhoods and their corridors often present opportunities for
urban renewal through the development of foreshoreways such as river
walks. Rivers also provide an easy means of disposing of waste water
and, in much of the less developed world, other wastes.

Fast flowing rivers and waterfalls are widely used as sources of
energy, via watermills and hydroelectric plants. Evidence of
watermills shows them in use for many hundreds of years, for instance
in
OrkneyOrkney at Dounby Click Mill. Prior to the invention of steam power,
watermills for grinding cereals and for processing wool and other
textiles were common across Europe. In the 1890s the first machines to
generate power from river water were established at places such as
CragsideCragside in
NorthumberlandNorthumberland and in recent decades there has been a
significant increase in the development of large scale power
generation from water, especially in wet mountainous regions such as
Norway.
The coarse sediments, gravel, and sand, generated and moved by rivers
are extensively used in construction. In parts of the world this can
generate extensive new lake habitats as gravel pits re-fill with
water. In other circumstances it can destabilise the river bed and the
course of the river and cause severe damage to spawning fish
populations which rely on stable gravel formations for egg laying.
In upland rivers, rapids with whitewater or even waterfalls occur.
RapidsRapids are often used for recreation, such as whitewater kayaking.[18]
Rivers have been important in determining political boundaries and
defending countries. For example, the
DanubeDanube was a long-standing
border of the Roman Empire, and today it forms most of the border
between
BulgariaBulgaria and Romania. The Mississippi in
North AmericaNorth America and the
RhineRhine in
EuropeEurope are major east-west boundaries in those continents.
The Orange and Limpopo Rivers in southern Africa form the boundaries
between provinces and countries along their routes.
Ecosystem
Main article:
RiverRiver ecosystem
The organisms in the riparian zone respond to changes in river channel
location and patterns of flow. The ecosystem of rivers is generally
described by the river continuum concept, which has some additions and
refinements to allow for dams and waterfalls and temporary extensive
flooding. The concept describes the river as a system in which the
physical parameters, the availability of food particles and the
composition of the ecosystem are continuously changing along its
length. The food (energy) that remains from the upstream part is used
downstream.
The general pattern is that the first order streams contain
particulate matter (decaying leaves from the surrounding forests)
which is processed there by shredders like
PlecopteraPlecoptera larvae. The
products of these shredders are used by collectors, such as
Hydropsychidae, and further downstream algae that create the primary
production become the main food source of the organisms. All changes
are gradual and the distribution of each species can be described as a
normal curve, with the highest density where the conditions are
optimal. In rivers succession is virtually absent and the composition
of the ecosystem stays fixed in time.
Chemistry
Main article:
RiverRiver chemistry
The chemistry of rivers is complex and depends on inputs from the
atmosphere, the geology through which it travels and the inputs from
man's activities. The chemical composition of the water has a large
impact on the ecology of that water for both plants and animals and it
also affects the uses that may be made of the river water.
Understanding and characterising river water chemistry requires a well
designed and managed sampling and analysis.
Brackish water
Further information: Brackish water
Some rivers generate brackish water by having their river mouth in the
ocean. This, in effect creates a unique environment in which certain
species are found.
Flooding
Main article: Flood

Flash flooding caused by a large amount of rain falling in a short
amount of time

Flooding is a natural part of a river's cycle. The majority of the
erosion of river channels and the erosion and deposition on the
associated floodplains occur during the flood stage. In many developed
areas, human activity has changed the form of river channels, altering
magnitudes and frequencies of flooding. Some examples of this are the
building of levees, the straightening of channels, and the draining of
natural wetlands. In many cases human activities in rivers and
floodplains have dramatically increased the risk of flooding.
Straightening rivers allows water to flow more rapidly downstream,
increasing the risk of flooding places further downstream. Building on
flood plains removes flood storage, which again exacerbates downstream
flooding. The building of levees only protects the area behind the
levees and not those further downstream. Levees and flood-banks can
also increase flooding upstream because of the back-water pressure as
the river flow is impeded by the narrow channel banks.
Flow
Studying the flows of rivers is one aspect of hydrology.[19]
Direction

Rivers flow downhill with their power derived from gravity. The
direction can involve all directions of the compass and can be a
complex meandering path.[20][21][22]
Rivers flowing downhill, from river source to river mouth, do not
necessarily take the shortest path. For alluvial streams, straight and
braided rivers have very low sinuosity and flow directly down hill,
while meandering rivers flow from side to side across a valley.
Bedrock rivers typically flow in either a fractal pattern, or a
pattern that is determined by weaknesses in the bedrock, such as
faults, fractures, or more erodible layers.
Rate
Main article: Streamflow
Volumetric flow rate, also known as discharge, volume flow rate, and
rate of water flow, is the volume of water which passes through a
given cross-section of the river channel per unit time. It is
typically measured in cubic metres per second (cumec) or cubic feet
per second (cfs), where 1 m3/s = 35.51 ft3/s; it is
sometimes also measured in litres or gallons per second.
Volumetric flow rateVolumetric flow rate can be thought of as the mean velocity of the
flow through a given cross-section, times that cross-sectional area.
Mean velocity can be approximated through the use of the Law of the
Wall. In general, velocity increases with the depth (or hydraulic
radius) and slope of the river channel, while the cross-sectional area
scales with the depth and the width: the double-counting of depth
shows the importance of this variable in determining the discharge
through the channel.
FluvialFluvial erosion
In the youthful stage;
V-shaped valleys: example.
RiverRiver Liffey, Dublin, Ireland.
When the river is subject to vertical erosion, deepening the valley.
Hydraulic actionHydraulic action loosens and dislodges the rock. The rivers load
further erodes its banks and the river bed. Over time, this will
deepen the river bed and create steeper sides which are then
weathered.
The steepened nature of the banks causes the sides of the valley to
move downslope causing the valley to become V-Shaped.
Waterfalls also form in the youthful river valley. example.
Powerscourt Waterfall, County Wicklow, Ireland.
Waterfalls usually form where a band of hard rock lies next to a layer
of soft rock (easier to erode). Differential erosion occurs as the
river can erode the soft rock easier than the hard rock, this leaves
the hard rock more elevated and stands out from the river below.
Hydraulic actionHydraulic action and abrasion are what erodes the soft rock and the
water to fall down to the river bed. A plunge pool forms at the bottom
and deepens as a result of hydraulic action and abrasion.[23]
SedimentSediment yield
SedimentSediment yield is the total quantity of particulate matter (suspended
or bedload) reaching the outlet of a drainage basin over a fixed time
frame. Yield is usually expressed as kilograms per square kilometre
per year.
SedimentSediment delivery processes are affected by a myriad of
factors such as drainage area size, basin slope, climate, sediment
type (lithology), vegetation cover, and human land use / management
practices. The theoretical concept of the 'sediment delivery ratio'
(ratio between yield and total amount of sediment eroded) captures the
fact that not all of the sediment is eroded within a certain catchment
that reaches out to the outlet (due to, for example, deposition on
floodplains). Such storage opportunities are typically increased in
catchments of larger size, thus leading to a lower yield and sediment
delivery ratio.

Main article:
RiverRiver engineering
Rivers are often managed or controlled to make them more useful, or
less disruptive, to human activity.

Dams or weirs may be built to control the flow, store water, or
extract energy.
Levees, known as dikes in Europe, may be built to prevent river water
from flowing on floodplains or floodways.
Canals connect rivers to one another for water transfer or navigation.
RiverRiver courses may be modified to improve navigation, or straightened
to increase the flow rate.

RiverRiver management is a continuous activity as rivers tend to 'undo' the
modifications made by people. Dredged channels silt up, sluice
mechanisms deteriorate with age, levees and dams may suffer seepage or
catastrophic failure. The benefits sought through managing rivers may
often be offset by the social and economic costs of mitigating the bad
effects of such management. As an example, in parts of the developed
world, rivers have been confined within channels to free up flat
flood-plain land for development. Floods can inundate such development
at high financial cost and often with loss of life.
Rivers are increasingly managed for habitat conservation, as they are
critical for many aquatic and riparian plants, resident and migratory
fishes, waterfowl, birds of prey, migrating birds, and many mammals.
See also

Definitions from Wiktionary
Media from Wikimedia Commons
News from Wikinews
Quotations from Wikiquote
Texts from Wikisource
Textbooks from Wikibooks
Learning resources from Wikiversity

Jeffrey W. Jacobs. "Rivers, Major World". Water Encyclopaedia.
Luna B. Leopold (1994). A View of the River. Harvard University Press.
ISBN 0-674-93732-5. OCLC 28889034. — a non-technical
primer on the geomorphology and hydraulics of water.
Middleton, Nick (2012). Rivers: a very short introduction. New York:
Oxford University Press. ISBN 9780199588671.